1. Field of the Invention
This invention relates to a high-power acceptable optical fiber and a method for fabricating the optical fiber.
2. Description of the Related Art
To accomplish super-long-distance optical transmission by using an optical fiber, it is necessary not only to reduce transmission loss of the optical fiber, but also to increase the intensity of signal light launched to the optical fiber.
As regards decreases in the transmission loss of an optical fiber, practical studies are being conducted. At present, in the case of optical fibers having GeO.sub.2 -doped silica cores, a fiber with a low transmission loss of 0.2 dB/km or less has been developed, while in the case of optical fibers having silica core and F-doped silica cladding, a fiber with a low transmission loss of 0.18 dB/km or less has been developed.
As regards increases in the intensity of light input to optical fibers, there have been significant advances in techniques, for example, the output-enhanced semiconductor laser and the erbium-doped optical fiber amplifier. However, it is known that the intensity of light which can be launched into an optical fiber is limited. If too intensive light of, for example, about 7 dBm or more is launched to an optical fiber, non-linear optical phenomenon will occur in which acoustic waves will be created in the fiber and will scatter lightwaves. Due to this phenomenon, known as "stimulated Brillouin scattering", most of the input light is reflected toward the light-input end of the fiber, not reaching the light-receiving end of the fiber. The phenomenon is explained in D. Cotter, "Observation of Stimulated Brillouin Scattering in Low-Loss Silica Fiber at 1.3 .mu.m" (Electronics Letters, vol. 18, No. 12, pages 495-496, 1982).
As is explained above, the amount of light which can be launched into an optical fiber is limited. To increase the transmission distance, it is necessary to suppress the stimulated Brillouin scattering. To suppress the scattering, it is necessary to increase the gain bandwidth of the stimulated Brillouin scattering spectrum, i.e., to cause nonuniform Brillouin frequency shift (the difference between the frequency of the input light and that of the scattered light).
A method for causing strain in an optical fiber is reported as a conventional technique for suppressing stimulated Brillouin scattering. This method utilizes the fact that application of strain to the optical fiber changes the Brillouin frequency shift.
This fact is disclosed in Kurashima Horiguchi, and Tateda, "Tensile Strain Effects on Brillouin Frequency Shift in Single-Mode Fibers Having Pure Silica and GeO.sub.2 -doped Cores" (IOOC' 89, 21C4-2). Further, as a method for utilizing the fact to increase the threshold value for the amount of light input to an optical fiber, over which the stimulated Brillouin scattering will occur, a technique for enlarging the gain bandwidth of the Brillouin scattering spectrum is reported in N. Yoshizawa, T. Horiguchi, T. Kurashima, "Proposal for Stimulated Brillouin Scattering Suppression by Fiber Cabling" (Electronics Letters, Vol. 27, No. 12, pages 1100-1101, 1991). In the method, optical fibers are helically stranded in a cable so as to change the strain caused in the axial direction of the fibers.
As is shown in FIG. 1, for example, the above-reported optical fiber cable employs optical fibers 11 of a double helix structure, extending in the axial direction, and the strain applied to the fibers 11 alternately changes from compressive strain to tensile strain, and vice versa. Thus, the Brillouin frequency shift varies in different portions of the fibers, increasing the gain bandwidth of the Brillouin scattering spectrum. Consequently, the threshold value for the amount of light increases, over which the stimulated Brillouin scattering will occur.
Moreover, a method for changing a residual stress in the axial direction of an optical fiber by changing the tension of the fiber during drawing the same is explained in Nozawa, Sakai, Wada, and Yamauchi, "Optical Fiber With Suppressed Stimulated Brillouin Scattering" (Proceedings of the 1991 of Institute of Electronics, Information and Communication Engineers of Japan Fall Conference, B-546). FIG. 2 shows an axial profile of tension on a Brillouin scattering-suppressed optical fiber consisting of a SiO.sub.2 core and an F-doped SiO.sub.2 cladding.
However, an optical fiber fabricated by the conventional methods for applying axial-directional strain to an optical fiber is liable to break. If the strain applied to a usual optical fiber increases from 0.1% to 0.2%, the factor of breakage per 1 km will become 10.sup.7 times higher. This is indicated in Yutaka Mitsunaga, Yutaka Katsuyama, Takakazu Kobayashi, Yukinori Ishida, "Method for Securing the Rigidity of Optical Fiber by Screening Test" (Transactions of the Institute of Electronics and Communication Engineers of Japan, Vol. J66-B, No. 7, 1983). To avoid such breakage, a technique for increasing the rigidity of an optical fiber by coating the fiber with a carbon film, must be employed in the conventional methods. In addition, since the optical fiber is kept in a particular strained state in the conventional method shown in FIG. 1, the cable structure is inevitably limited. It is therefore very difficult to employ the structure in the actual optical fiber line.